Conventionally, a manufacturing method and its apparatus of a steering rack (for example, a variable steering rack of a complicated shape) for forging and forming in one process by using a closed type die, and minimizing the deburring work of a product after the forming process have been proposed by the present applicants and put in practical use (see, for example, patent document 1).
The manufacturing apparatus of the steering rack disclosed in patent document 1 is designed to forge and form by using a closed type die formed to absorb an escape of materials due to plastic deformation, depending on the shape of the steering rack to be formed, and since the forging and forming operation is carried out in one process, plastic changes in the circumferential direction hardly appear as burrs, and the deburring process of the product after forging and forming can be saved, and the manufacturing cost can be lowered.
However, in this manufacturing apparatus of the steering rack, since the upper die and the lower die are individually made of a die of integral shape, from the beginning of the process of forming a tooth portion in the steering rack material, it is not possible to confine the circumference at both end portions in the axial direction of the steering rack material by the upper die and the lower die, and the wall of a tooth portion forming part of the steering rack material escapes to the end portion side in the axial direction of the steering rack material, and burrs may be formed at this position, and the wall thickness of the tooth portion forming part may be insufficient, and the yield is lowered when manufacturing products, and the forming precision of the products becomes inferior.